The National Scene - Monday, September 17, 2007 13:15 - 0 Comments

Potential Flaw Seen in Design of Fallen Bridge

Investigators in Minnesota have found what may be a design flaw in the bridge that collapsed in that state resulting in the loss of lives. The flaw in the steel parts that connected girders raises safety concerns for other bridges around the country. As a result, the Federal Highway Administration has urged all states to take extra care with how much weight they place on bridges of any design when sending construction crews to work on them. Crews were doing work on the deck of the Interstate 35W bridge when it gave way. It should be noted that the National Transportation Safety Board’s investigation is months from completion. Investigators are working to confirm the design flaw in the so-called gusset plates. They are attempting to determine what, if any, role the plates had in the collapse of the bridge.

The investigators have indicated that they considered their preliminary findings to be a potentially crucial discovery and also a safety concern for other bridges. Gusset plates are used in the construction of many bridges, not just those with a similar design to the one in Minnesota. Since the collapse, the concern among investigators has focused on “fracture critical” bridges, which can collapse if even a single part fails. But thus far, neither the safety board nor the federal Department of Transportation has singled out any particular design of bridge in raising its new concerns about gusset plates and the weight of construction equipment. Concerns about the plates emerged from scrutiny of the vast design records related to the steel truss bridge. In

A friend of mine in the steel business told me that gusset plates are the steel connectors used to hold together the girders on the truss of a bridge. On the Minnesota bridge, completed in 1967, there would have been hundreds of them, according to Minnesota officials. Safety board investigators are “verifying the loads and stresses” on the plates as well as checking what they were made of and how strong they were. It has been reported that the use of these plates is common in bridges. It’s a way to attach several girders together.

I understand that a consultant, hired by the State of Minnesota in the days after the collapse to conduct an investigation of what had gone wrong, discovered the potential flaw. According to Federal authorities, one added stress on the gusset plates may have been the weight of construction equipment and nearly 100 tons of gravel on the bridge, where maintenance work was proceeding when the collapse occurred. A construction crew had removed part of the deck with 45-pound jackhammers, in preparation for replacing the two-inch top layer, and that may also have altered the stresses on the bridge, according to experts. It will take a great deal of more hard work to determine exactly what caused the bridge to collapse. The tragedy has certainly been a wakeup call for governmental officials across the country.

Sources: New York Times and Associated Press